Interference removal in pointing device locating systems

ABSTRACT

An apparatus and a method for determining the location of a pointing device in the vicinity of a set of receivers able to receive one or more locating signals transmitted through a medium. The method includes receiving at a receiver a signal that includes a locating signal and an interfering signal, determining an estimated interference signal indicative of the interfering signal included in the signal received, determining a signal indicative of the difference between the received signal and the estimated interference signal, and using the signal indicative of the difference to compute the location of the pointing device on a surface near the set of receivers. One version uses a separate receiver from which to determine the estimated interference signal, while another version uses the received signal at a time when there is expected to be no locating signal present in order to determine the estimated interference signal. An adaptive filter computes the estimated interference signal.

RELATED APPLICATION

The present invention claims priority of U.S. Provisional PatentApplication No. 60/564,909, filed Apr. 23, 2004 to inventors Weaver etal., titled “INTERFERENCE REMOVAL IN POINTING DEVICE LOCATING SYSTEMS,”and hereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to pointing device locating systems thatuse signal transmitters and receivers, and in particular—to such systemsoperating in the presence of interfering signals such as noise.

BACKGROUND

There are known systems for determining the position or motion of amovable device, such as a stylus, a pen, a whiteboard marker, or otherpointing device over a plane, which use signals transmitted through amedium, such as air, between one or more transmitters and one or morereceivers, wherein the position is determined by proper processing ofreceived signals; such systems are sometimes also referred to as activetracking systems or as location transcription systems and willcollectively be referred to herein as locating systems. In one versionof a locating system, depicted schematically in FIG. 1A, a transmitter 1that is attached to a pointing device 2 emits a signal of repeatedultrasonic pulses and this signal is received by two or more receiversat known locations, e.g., attached to a stationary frame near an activesubstantially planar area. FIG. 1A shows two ultrasonic receivers 3attached to a frame 5. The pointing device 2 also emits infrared pulsesthat are received by an infrared detector 4 on the frame 5. The relativetimes of arrival of the signals at each receiver are detected and fromthese values the distances from the transmitter 1 to the receivers 3 and4 are determined and hence, by triangulation, also the position of thepointing device 2 with respect to the stationary frame. Thus, an activearea 8 is defined wherein the location system can determine the locationof the pointing device 2. A transmitter 9 transmits the so-determinedpositions, e.g., to a matching receiver 10 on a computer such as alaptop computer 11. Transcriptions of a set of locations, e.g., a line12, is sent and may be stored in the computer 11.

FIG. 1B shows an example of the pointing device 2, that may include atip 15 that may be a marking element. The pointing device includes abody 13 and a pressure sensitive switch 14 that starts transmission ofthe signals from the transmitters when the tip is pressed against theplanar surface. One or more buttons 16 may be included in the pointingdevice to provide indications to the receiver array containing receivers3 and 4.

U.S. Pat. No. 6,335,723 to Wood, et al., entitled “TRANSMITTER PENLOCATION SYSTEM,” assigned to the assignee of the present invention andincorporated herein by reference, discloses one such system for locatingthe position of a pen. It is noted that, in addition to a relativelyslow-propagation signal, typically an ultrasonic signal, Wood, et al.also describes a version that includes on the pointing device atransmitter such that there also is transmitted and received afast-propagation signal, typically an infrared signal, which serves toprovide the ultrasonic receivers with a time base for calculating therespective propagation times of the slow signal. Both transmittedsignals are structured as a mutually synchronized train of pulses. It isfurther noted that in an alternative embodiment, any of the signals mayalso serve to carry supplementary information, such as pen color, and/orother pen parameters.

U.S. Pat. No. 6,414,673 to Wood, et al., entitled “TRANSMITTER PENLOCATION SYSTEM,” assigned to the assignee of the present invention andincorporated herein by reference, discloses another such system. Here,basically, the directions rather than the times of arrival of thetransmitted signal at each receiver are detected and hence the locationof the pen is calculated. In one embodiment, the time of arrival at areceiver is also detected. U.S. Pat. No. 6,184,873 to Ward, et al.,entitled “PEN POSITIONING SYSTEM,” assigned to the assignee of thepresent invention and incorporated herein by reference, discloses avariation of the aforementioned systems, wherein there are two outputelements attached to the pen, at given different distances from its tip,each transmitting at a unique ultrasonic frequency. The two frequenciesare processed separately at the receivers, to determine the respectivepositions of the output elements; from these, the position of the tip ofthe pen is calculated.

Further variations of the aforementioned systems are possible. Forexample, there may be a single receiver, to determine a position along asingle axis, or the number of receivers may be greater than two—toincrease the accuracy of triangulation, to determine the position alongthree dimensions, or to increase the active area. As another example,one or more receivers may be attached to the movable device whiletransmitters are attached to the stationary frame. As yet anotherexample, the medium, which in the aforementioned systems is air, may bea vacuum or may consist of any other substance, whether gaseous, liquidor solid; concomitantly, the signaling modality, besides being acousticor electromagnetic, as in the aforementioned systems, may be any othertype, such as surface acoustic waves. The pointing device itself may beof any shape and may serve any purpose in addition to just beinglocatable and its motion may be effected by a human operator or by amachine. The pointing device typically is movable.

FIG. 1C shows a typical functional block diagram of the sensor arraythat includes sensors 3 and 4 and that includes processing of thesignals received by the sensors. A signal conditioner (17A, 17B, 17C)includes filtering of signals that are out of the expected frequencyrange, and includes anti-alias filtering. The pen 2 simultaneouslytransmits an ultrasound pulse and an infrared (IR) pulse. The IR pulseis assumed to travel much faster than the ultrasound pulse, and hence isreceived first at the IR receiver 4. An analog-to-digital converter(ADC) (18A, 18B) initially converts the infrared pulses and thesedigitized infrared signals are input serially to a processor, in oneembodiment, a DSP device 19. In particular, the data is input to thememory of the DSP device 19. To reduce costs of additional ADCs, aswitch then switches the input of the ADC to receive signals from one ofthe ultrasound receivers. Another ADC also receives signals from thesecond ultrasound receiver. Thus, after the IR pulse is received,digitized ultrasound signals are received, digitized, and input to thememory of the DSP device 19 for further processing.

In one embodiment, the program for such processing is kept in a flashmemory coupled to the bus of the DSP device. The processing determinesthe time of arrival of the ultrasonic pulses received at the tworeceivers relative to that of the infrared signal. From these times ofarrival and the known positions of the ultrasound receivers, the DSPdevice calculates the location of the transmitter at the time theultrasound pulses were transmitted.

In one embodiment, the location information is transmitted, e.g., viaBluetooth technology or a USB cable, to another device such as acomputer 11.

Common to all such systems, which the present invention addresses, isthe presence of at least one transmitter and at least one receiver, atleast one of which is attached to a movable device; any transmittertransmits through the medium at least one useful signal, which isreceived by at least one receiver and subsequently processed; theprocessing of one or more of the received signals leads to adetermination of the current position or velocity of the movable devicealong at least one dimension. The useful signal may be either in a slowpropagating mode, such as acoustic (usually ultrasonic) waves, servingto manifest propagation time that is proportional to, and thusindicative of, the distance traveled in the medium, or it may be in afast propagating mode, such as electromagnetic waves (usually in the IRrange), serving, for example, to provide time reference; in either roleit will also be referred to herein as a locating signal, though it mayoptionally have other information encoded thereto. A locating signal isgenerally characterized by a given carrier frequency, as is known insignaling practice.

It is often the case that there is an interfering signal present in themedium or transmitted therethrough, such a signal emanating, e.g., froma source other than any of the system's transmitters, and such a signalreceived by any of the receivers in addition to the useful signal. Theinterfering signal may, for example, be electromagnetic induction frompower lines and devices, light from high-frequency lighting devices oran ultrasonic signal from another source, such as a motion detector.Such interfering signals may degrade the results of the correspondingprocessing, possibly causing an error in the determined position orvelocity or even making such determination altogether impracticable.Generally, locating and tracking systems require a relatively highdegree of accuracy and resolution—typically 1:1000 or better—andtherefore even relatively low levels of interfering signals may bedeleterious. Interfering signals may be regarded as noise. In thisdescription, noise, interference, interfering noise, and so forth areall used to mean the signal or signals interfering with the locatingsignals.

Now, if the interfering signal is clearly distinguishable from theuseful signal—for example, by having all frequencies substantiallydifferent from the useful signal's carrier frequency or by occurringwithin time periods distinct from those in which the useful signaloccurs—then the component of the received signal due to the interferingsignal may be removed or sufficiently reduced, using filteringtechniques known in the art. If, however, the interfering signal hasfrequency components close, or identical, to the useful signal'sfrequency and if it occurs substantially during time periods at whichthe useful signal occurs, no such filtering is effective for suchcomponents. It thus is desirable and would be useful to have a methodand apparatus to reduce interfering components in the received signal ina locating system especially in such cases as last discussed.

The inventors have found, for example, that for infrared detectors,fluorescent lights often emit interference in the same frequency rangeas the infrared location signals used in location determining systems.Furthermore, ultrasound motion detectors often produce ultrasoundsignals that are in the same frequency range as the ultrasound locationsignals used in location determining systems, and that are so strong asto reduce the accuracy of such systems, possibly even rendering thelocation determining system unusable in the presence of theinterference.

Thus there is a need in the art for a method for reducing the amount ofinterfering noise in locating signals used in location determiningsystems.

SUMMARY

One aspect of the present invention is a method to reduce a noisecomponent, relative to the useful component, in a signal received by areceiver from the transmission medium in a locating system, even in thecase that the noise and the useful component occur during common timeperiods and have a component in the same frequency range. Basically, themethod calls for producing, from a received signal, an estimatedinterference signal that is indicative of the noise component and thensubtracting the estimated interference signal from the signal receivedfrom the medium. The result of the subtraction is subsequentlyprocessed, as in the corresponding conventional system, to serve theintended purpose of the useful signal.

In one group of embodiments of the invention, particularly applicable tocases in which the locating signal is intermittent, e.g., formed as atrain of pulses, the estimated interference signal is produced from thesignal received by the same receiver that receives the locator signal,but a certain time interval earlier. In one embodiment, the signalreceived earlier is delayed by the certain time interval and filtered byan adaptive filter, to produce the estimated interference signal. Inanother embodiment, suitable in the case that an interfering signal hasa substantial amount of its energy in a single frequency, the signalreceived earlier is analyzed as to its frequency, phase and amplitudeand an estimated interference signal is generated accordingly.

Disclosed herein is a method for determining the location of a pointingdevice in the vicinity of a set of receivers able to receive one or morelocating signals transmitted through a medium. The method includesreceiving at a receiver a signal, which includes a locating signal andan interfering signal, determining an estimated interference signalindicative of the interfering signal included in the received signal,determining a signal indicative of the difference between the receivedsignal and the determined estimated interference signal, and using thedetermined signal indicative of the difference to compute the locationof the pointing device on a surface near the set of receivers.

Other aspects and features are described below and in the claimsattached hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with the help of the followingdrawings.

FIG. 1A is a diagram of a typical locating system. It is marked “PriorArt,” but is not prior art when it includes aspects of the presentinvention.

FIG. 1B is a diagram of a typical pointing device of the locating systemshown in FIG. 1A. Such a pen may be prior art.

FIG. 1C is a functional block diagram of the receiving part of thelocating system shown in FIG. 1A. It is marked “Prior Art,” but is notprior art when it includes aspects of the present invention.

FIG. 1D shows a simplified block diagram of one embodiment of theinvention that performs interference reduction using adaptive filers onthe ultrasonic received signals using the ultrasonic receivers, and afilter and infrared receiver to carry out interference reduction on theinfrared signal.

FIG. 2A shows the basic digital processing logic in an exemplary priorart locating system that uses three receivers, the first two of whichreceive ultrasound locating signals and the third of which receives aninfrared timing signal.

FIG. 2B is a simplified block diagram of a generic position determiningsystem.

FIG. 2C is a simplified block diagram of an analog position determiningsystem.

FIG. 2D is a simplified block diagram of a digital position determiningsystem.

FIG. 2E is a simplified block diagram of a hybrid analog and digitalposition determining system.

FIG. 3 is a block diagram of modified digital processing of receivedsignals in the system of FIG. 1, illustrating principles of theinvention.

FIG. 4A is a block diagram of additional digital processing of areceived signal according to a first embodiment of the invention

FIG. 4B is a schematic diagram of an embodiment of a FIR filter that canbe used in the processing shown in FIG. 4A.

FIG. 4C shows one embodiment of an adaptive filter structure for thefilter.

FIG. 4D shows the processing used in one embodiment to reduce theinterference in the infrared channel.

FIG. 5 is a block diagram of additional digital processing of a receivedsignal according to a second embodiment of the invention.

FIG. 6 is a block diagram of the additional digital processing of areceived signal according to a third embodiment of the invention.

FIG. 7 is a timing diagram, illustrating typical signals that occur inone or more embodiments.

FIG. 8A shows a simplified block diagram of one embodiment of theinvention that performs interference reduction using adaptive filers ononly the ultrasonic received signals.

FIG. 8B shows one embodiment of an adaptive interference reduction unitfor an ultrasonic signal using an adaptive filter that includes a tappeddelay line that provides a delay of 121 units.

DETAILED DESCRIPTION Location Determining Systems

One version of the basic digital processing logic in an exemplarylocating system of prior art, such as that depicted in FIGS. 1A and 1C,is presented schematically by the block diagram of FIG. 2A. In this casethere are three receivers (not shown), the first two of which receiveultrasound locating signals and the third of which receives an infraredtiming signal. For purposes of explaining the invention, the switch andthe ADCs of FIG. 1C are not shown, and it is therefore assumed that thesystem operates as if three signals are separately input. The tworeceived ultrasound signals after digitization and the infrared signalafter digitization are input to the digital processor as signals US1,US2, and IR, respectively. The locator processor 20 shown within thebroken line box includes two counters 22, one for each of the two inputlocating signals. Each counter counts supplied clock pulses, then isreset to zero by each detected pulse in the timing signal (IR) input andstops counting upon the detection of a pulse in the correspondinglocating signal input. The value from each counter is fed to acorresponding distance calculator 24. The outputs from the two distancecalculators are fed to a coordinates calculator 26, whose output formsthe output of the system.

Note that FIG. 2A is a simplified description, and does not, forexample, describe the mechanism used for actually determining the timesof arrival of the ultrasound signals from their digitized form. However,such a description is sufficient for the purpose of describing aspectsof the present invention.

Note also that FIG. 2A only deals with the aspects of determining thelocation. The location determining system may be analog, digital, or acombination of analog and digital. FIG. 2B, for example, shows a simplegeneric location determining system that accepts two signals denoted u1and u2 and produces a set of coordinates denoted c that represent themeasured position of the pointing device. FIG. 2C shows in simplifiedform a design where such a location determining system is to receiveanalog signals denoted u1(t) and u2(t), perform processing on u1(t) andu2(t), where t denotes time, and periodically update a position denotedc(t), which is produced by the location determining system as one ormore analog signals. FIG. 2D shows in simplified form a design whereinsuch a location determining system receives digital signals denotedu1(k) and u2(k) at times denoted t(kT), where k is an integer index, andT is a time period, and determines coordinate information c(N) at timest(NT′), where again N is an index and T′ is a time period. Usually theperiod T′ of the times t(NT′) is such that the times are spaced furtherapart in time than times t(kT). Thus, the location determining systemcan consider many samples of u1(k) and u2(k) before updating themeasured position c(N). FIG. 2E shows the preferred method of locationdetermining, which is a hybrid of analog and digital processing. Signalsu1(t) and u2(t) are conditioned using analog circuits. The signals arethen periodically sampled by analog to digital converters (ADCs) andthus converted into digital signals u1(k) and u2(k) before being inputto the location determining system. The present invention can be appliedto any of these different embodiments of location measurement systems aswell as to other hybrids of digital and analog processing systems.

FIG. 7 shows a set of signals pertaining to any one of the counters andcorresponding received ultrasound signals. The timing signal (Line 1denoted IR) includes a train of regularly spaced pulses, wherein theperiod of such pulses forms a measuring cycle. The length of themeasuring cycle is chosen to exceed the longest possible propagationtime through the medium of the locating signal over the active area,i.e., the area in which location of a movable device is to bedetermined. Each ultrasonic locating signal includes a train of pulses,in one embodiment spaced identically in time to the timing signalpulses. Thus, when a corresponding ultrasound transmitter and infraredtransmitter transmit, the locating signal pulses are essentiallysynchronous with the infrared timing signal pulses. Upon reception,however, the locating signal pulses lag the IR by a certain amount oftime, as seen in Line 2—denoted US—in FIG. 7. Assuming infrared travelsessentially instantaneously, this time is the propagation time of theultrasound signal through the medium, and thus is proportional to thedistance from the transmitter to the corresponding receiver assuming aconstant propagation velocity in the medium, and line-of-sightpropagation. It is a measure of this propagation time that the counter22 counts. Depending on the actual present distance between thetransmitter and the receiver, the value of the propagation time may varyover a given range, depicted in Line 3 of FIG. 7 as the “US sensingwindow”. At the end of the sensing window, the value of the count is fedto the corresponding distance calculator 24, which multiplies it by afactor proportional to the signal propagation speed, to arrive at adistance value. The distance values from the two distance calculators 24are fed to the coordinates calculator 26. In one embodiment, thecoordinates calculator 26 is calibrated using knowledge of the positioncoordinates of the receivers. Using the outputs of the two distancecalculators 24, the coordinates calculator 26 proceeds to calculate, bytriangulation, the coordinates of the transmitter indicative of theposition of a reference point on the movable device.

The entire processing logic, as described, for example, above and shownas 20 in FIG. 2, is prior art and will be referred to as the locator. Itis typically carried out by a digital signal processor (DSP) device, asdepicted in FIG. 1C, but may also be embodied in other ways. It will,furthermore, be appreciated that the locator may also incorporate otherforms of logic and that the sole purpose of describing it here is toserve as an exemplary context within which to explain the presentinvention.

Noise Reduction

As mentioned in the Background section, there are situations in whichany of the signals entering the locator 20 may also contain a noisecomponent. For example, motion detectors may interfere with theultrasound signals. As another example, fluorescent lights may interferewith the infrared signals. One aspect of the invention is to reduce orremove the noise component from the signal, e.g., prior to calculationsbeing performed thereon for the purpose of determining location. Withoutloss of generality, it may be assumed that such interfering noise iscontinuous and is present also during the expected occurrence time ofthe respective signal pulses. It further may be assumed that theinterfering noise includes frequencies very near or at those of thecorresponding carrier (IR or ultrasound). Any noise of componentsoutside this frequency range may be appropriately reduced or removed byconventional filtering, as known in the art.

The basic principles of the invention are illustrated by the simplifiedblock-diagram of FIG. 3, which depicts additional blocks in each of theinputs to the locator 20, which may be identical to the locator of FIG.2A or any other one of prior art. In each input line there is seen asumming device (an adder), shown respectively as 331, 332, and 333 foreach of the respective signal paths, with a positive input and anegative input, thus functioning effectively as a subtractor. The termsan adder having one positive and one negative input, and a subtractorare used synonymously herein. One input, here the positive input,receives the corresponding signal from the receiver and the other input,here the negative input, receives what is called herein an “estimatedinterference signal” from a corresponding interfering noise estimator,shown as 351, 352, and 353 for each of the signal paths. One aspect ofthe invention is that the estimated interference signal is a closeapproximation of the noise component expected in the received signal andthus the subtraction action effectively reduces or eliminates the noisecomponent, leaving only the signal used for the locator system. Thequality of the estimated interference signal determines how effectivelythe noise component is cancelled in the output from summing device 331,332, or 333.

Disclosed herein are several embodiments of the noise estimator 351,352, and 353. In general, if such noise estimators are included for allsignals, e.g., for the two ultrasound signals and the infrared signal,the ones 351 and 352 for the ultrasound signals, may be different thanthat, 353, for the infrared signal.

In general, the noise estimator and the summing device may be embodiedas analog circuitry, in which case they would be positioned prior to thecorresponding ADC (FIG. 1C), or as digital processing modules, includingcode blocks in a programmable processor. In one embodiment, a noiseestimator 353 for the infrared signal, is included and implemented as ananalog circuit, while those 351 and 352 for the ultrasound signal indigital processing modules. In another embodiment, all estimators 351,352, and 353 are implemented in digital form. The latter embodiment ispreferred because it allows incorporating all modules in the samedigital processor that serves for the locator 20.

Various embodiments of the invention will now be described in greaterdetail, in terms of any one of the signals input to the locator. It willbe appreciated that the invention is thus applicable to any of thesignals, as well as to any other similar signal.

Use of Separate Reference Signal Receiver(s)

Reference is now made to FIG. 4A, which depicts schematically a firstembodiment of the invention. In this embodiment, an additional receiver42 is provided, to be termed an “interference receiver” or a “referencesignal receiver,” which is placed so as to receive a signal (a referencesignal) that in this version is separate from those signals used for thelocator signal. The interference receiver is instead configured tomeasure a signal from which the interfering signal or an estimatethereof can be determined. The interference receiver may measure theinterference directly or may measure a quantity that is correlated tothe noise component to be removed, for example a lower amplitudeversion, and/or time-shifted version of the actual noise componentpresent in the signal received for location determining. All suchversions of the reference signal have in common that each is correlatedto the interfering signal that appears with the location signal.

In the digital embodiment, the receiver 42 is assumed to include anyappropriate analog signal conditioning and appropriate conversion to adigital version, similar to those of the other receivers (e.g., asdepicted in FIG. 1C).

The received interference signal (the reference signal) is applied to afilter 43. In one embodiment, the filter 43 is pre-set, e.g., chosen atthe factory to appropriately filter the received interference signal,also called the received reference signal, to produce an estimatedinterference signal. In another embodiment, the filter can be settable.In one embodiment, the filter can be a simple amplitude adjuster toadjust for gain differences between the gain the interference componentin the locator signal experiences, and the gain the reference signalexperiences. In another embodiment, one or more delay elements may beincluded. In general, the filter is defined by a set of one or morefilter parameters.

In one version, several sets of filter parameters are provided. The setto use for the filter is according to a selection criterion.

One such selection criterion is now discussed.

Denote by y(k) the values of the received signal, s(k) the desiredsignal, and n(k) the interfering noise. Then assume the noise isadditive, i.e.,y(k)=s(k)+n(k).

Denote the estimated interference signal as d(k), and the error as e(k).Thene(k)=y(k)−d(k)=s(k)+n(k)−d(k).

Denote expected value, i.e., the mean value, by E{.}. The mean of thesquared error, assuming that the desired signal s(k) is uncorrelatedboth to n(k) and to d(k) is approximatelyE{e ²(k)}≈E{s ²(k)}+E{[n(k)−d(k)]²}.

Thus, by minimizing E{e²(k)}, one obtains an output that minimizes theamount of interference. One estimate of E{e²(k)} is the sum of squarederrors, e.g., over some number, denoted M, of samples. Thus, denoting bye(k) the vector of the M error signals, i.e.,e (k)=e(k), e(k−1), . . . , e(k−M+1),

one criterion for selecting the set of filter parameters is to selectthe set that minimizes the sum of the squares of e(k)'s, beingΣ_(i=0 to M−1) [e ²(k−i)]= e (k)· e (k), where · denoted the innerproduct.

See below for more description of a least squares method, appliedhowever to a different embodiment in which the reference signal isobtained from the same receiver as that receiving the location signal.

Thus, one embodiment of the invention selects the set of filterparameters that minimize the amount of interference. Other criteria arepossible and other embodiments use one or another of such othercriteria.

Use of an Adaptive Filter

FIG. 4B shows a second embodiment, in which the filter that filters thereceived reference signal that provides an estimated interference signalis a digital adaptive filter 44. Digital adaptive filters are known inthe art and have been extensively described in the literature. Asuitable type of an adaptive filter is the Finite Impulse Response (FIR)adaptive filter with adaptively determined coefficients (weights), whichis readily realizable in a DSP, as well as other digital processors andcircuits. The weights of the FIR filter are determined by a weightdeterminer 46 according to one of several well known adaptive filterweight determining methods in order to reduce the amount of interferencein the output, using as an error signal, the difference between theinput signal and the estimated interference signal out of the adaptivefilter. The purpose of the adaptive filter is to make the filteredversion of the interference signal similar to the actual interference inthe input signal.

One adaptive filter structure for filter 44 is illustrated in FIG. 4C,and includes a series of delay sections 442, through which the signalpasses, variable coefficient units 444 that provide variable gain orattenuation, and that tap the signal at corresponding delay points.These coefficient units 444 are typically implemented by multipliers,and provide to each such delayed version of the input a given relativeweight. Each weighted delayed version is summed by an adder-accumulator446, and there are a series of such adder-accumulators 446, such thatthe final adder accumulator provides the output as a weighted sum ofdelayed versions of the input signal. A DSP device typically includessets of multipliers and add-accumulate units for carrying out suchfiltering operations. The output of the filter is the estimate of theinterference denoted d(k) that estimates the interference in the inputsignal, and such estimate d(k) is applied to a corresponding input of acorresponding adder 33, the other input being the normally receivedsignal denoted y(k), such as US1 in FIG. 4A that is assumed to includeinterference. The output of the adder 33, denoted e(k), varies as thedifference between the normally received signal y(k) that may containinterference, and the estimated interference signal d(k) that estimatesthe interference, and that is obtained by adaptive filtering. Suchoutput of adder 33 is to be input to the locator 20 (FIG. 3), and isalso applied to an adaptive weight calculator 46 in the adaptive filter44.

The adaptive weight calculator 46 monitors the difference signal shownas e(k) and accordingly calculates the adaptively determined weights,shown as weights w₀(k), w₁(k), . . . , w_(N)(k) for N delays andexpressible as an N+1 weight vector w(k) at discrete time instantdenoted by index k.

Thus, the output e(k) ise(k)=y(k)−d(k)= w (k)· r (k)

where the vector r(k)=vector of r(k), r(k−1) . . . r(k−N), the presentvalue and N past values of the reference signal r(k), and · representsan inner product (a dot product). This is a vector form of expressingthe convolution operation of the FIR filter.

The weight determining unit 46 adjusts the weight vector w(k) accordingto previous values, to the previous error value, and to previous valuesof the interference signal. Thus, the weights in the filter are varied,as indicated by the arrow across the filter block in FIG. 4A, thuscompleting an adaptive correction loop. The iterative weight calculationis preferably according to one of several variants of the Least MeanSquare (LMS) algorithm, according to which the weights adapt towards theweights that minimize the mean square error between a desired signal andthe output signal

As above, suppose y(k)=s(k)+n(k), where n(k) is the interference in theinput signal y(k) and s(k) is the desired signal, and define the errorase(k)=y(k)−d(k)=s(k)+n(k)−d(k).

As was shown above, E{e²(k)}≈E{s²(k)}+E{[n(k)−d(k)]²}. Thus, byminimizing the mean squared output of the summer, E{e²(k)}, one obtainsan output that minimizes the amount of interference.

While it is known that methods such as the steepest descent methodsiterate towards achieving the minimum by moving in the direction of thevector gradient, the purpose of various weight calculation methods,e.g., the LMS method, or its normalized variant, NLMS, is to approximatethe gradient by an estimate determined from the error (output of theadder) and the input interference signal.

Denote the k'th iteration of the weight vector by w(k).

In one embodiment, the next weight vector is expressed by the followingformula:w (k+1)= w (k)+μe(k) r (k)where r(k) is the vector of r(k), r(k−1), . . . , r(k−N), the presentvalue and N past values of the input to the adaptive filter, i.e., thereference signal obtained by the interference receiver. k is an indexindicating a sample in time, w(k) is a vector indicating the set ofweight values at the k'th sample time, μ is a parameter that controlsthe adaptation rate, also called the gain, the step size, or theforgetting factor, and e(k) is the desired output (difference) signalvalue at the current sample point k.

In one embodiment, the normalized variant of the LMS method—theNormalized Least Mean Squared (NLMS) method—is used. The forgettingfactor μ that controls the adapting rate is scaled according to theinverse square of the signal energy. That is, for a number N of signals,is scaled according to 1/[N Input signal power]. In one embodiment,denoting r(k) as the vector of the reference input to the adaptivefilter, the adaptation is according to:w (k+1)= w (k)+μe(k) r (k),whereμ=μ′/{λ r (k)∥² }=μ′/[r (k)· r (k)].

The parameter μ′ is selected to be between 0 and 2, usually less than 1.

In yet another version of the NLMS method,μ=μ′/[a+r (k)· r (k)],where a is a positive constant.

Other alternate versions of the LMS method also are known, such as thesign LMS, in which the adaptation is:w (k+1)= w (k)+μe(k)sgn{ r (k)},where sgn{ } is +1 or −1 according to whether r(k) is positive ornegative.

Other adaptive filters also are known and may be used.

Note that a single reference receiver may be used for correcting thesignals of all the normal receivers of the same modality—e.g., the twoultrasonic receivers. It is also noted that the blocks “Interferencereceiver” 42, “Adaptive filter” 44 and “adaptive weight calculator” 46in FIG. 4A collectively constitute, in effect, any one of the blocksgenerically named “Noise estimator” 351, 352, or 353 in FIG. 3.

Differential Detectors

One version of the embodiment described above is used for reducing noisein the infrared channel.

The inventors discovered that fluorescent lights, or components used toproduce the electric signals for fluorescent lights, often produceinterference to the infrared signal. However, the inventors alsodiscovered that the visible light component from such fluorescent lightis correlated to the infrared component. Thus, in one embodiment, aseparate receiver is used to measure a reference signal to determine theinterference estimate for the infrared signal. In one embodiment, such areference signal is produced by physically placing a physical IR filteraround a receiver to filter out the IR component to produce a referencesignal that is relatively free of the useful IR signal. FIG. 1D showsone arrangement for so detecting and storing digital samples denotedr(k) of such a reference signal. The two sensors 3 are each forultrasound. The signals 4 and 4′ are for electromagnetic radiation.Sensor 4 is for IR radiation as in FIG. 1C. Sensor 4′ includes a filter6 that reduces radiation in the IR range. In one embodiment, a thin-filminfrared filter, made by UNAXIS OPTICS of Golden Colorado is used aroundthe sensor to significantly reduce the near-infrared that includesprocessing of the signals received by the sensors.

As in FIG. 1C, a signal conditioner includes the filtering of signalsthat are out of the expected frequency range, and includes anti-aliasfiltering. The two analog-to-digital converters (ADC) initially convertthe electromagnetic radiation pulses and these digitized electromagneticradiation signals are input serially to a processor, in one embodiment,a DSP device 19. In particular, the data is input to the memory of theDSP device 19.

While in one embodiment, separate ADCs may be used for each of theelectromagnetic energy signals, in the embodiment shown in FIG. 1D, twoswitches then switch the inputs of each of the ADCs to receive signalsfrom a respective one of the ultrasound receivers. Thus, initially, onechannel stores samples of the IR signal, while another stores samples ofthe reference signal to use for reducing the interference in the IRchannel. Then, after the IR pulse and the electromagnetic referencesignal are received, digitized ultrasound signals are received,digitized, and input to the memory of the DSP device 19 for furtherprocessing.

FIG. 4D shows the processing used in one embodiment to reduce theinterference in the infrared channel. The stored reference signal is inthis case the digitized signal of the sensor that received the filteredradiation, i.e., with very little IR, while the other channel includesthe IR pulses emitted by the pointing device, and also any interferingnoise. The adaptive filter adapts using the reference signal to producea relatively interference-free signal to use for the infrared channel ofthe pointing device.

Using the Same Receiver for Interference and for Location Signals

A different additional set of embodiments is now described. Common tothe following additional embodiments is that the received interferencesignal (also called reference signal) r(k) from which is obtained theestimated interference signal d(k), is derived from the same receiverthat supplies the normal signal denoted y(k), and that such derivationinvolves some time-shift or delay. The aim is again to obtain anestimated interference signal d(k) that is maximally indicative of thenoise component in the normal signal, but, while in the first embodimentshown in FIGS. 4A and 4B, this is achieved by an independent receiver ofthe interference (reference) signal r(k), the principle used in theseadditional embodiments is to obtain r(k) from y(k) itself, e.g., bydelaying the signal y(k) from one and the same receiver.

As already explained and as depicted in FIG. 7, the useful signals oflocating systems are generally characterized by relatively narrow pulsesthat repeat regularly. Thus there are, in between the pulses, intervalperiods, in which no useful signal energy is transmitted. On the otherhand, interfering signals are usually continuous and in any case maytransmit energy also during these interval periods.

Thus, one set of embodiments uses the interval periods in which nouseful signal pulse is expected for training the adaptive filter.Another uses the interval periods in which no useful signal pulse isexpected to predict the estimated interference signal to use when theuseful signal is present.

As a first example, in the case of a timing signal, transmitted by IRpulses, the entire interval between successive IR pulses may beconsidered as empty of useful signal. This is a suitable time forobtaining an estimate of the interference signal, e.g., by predictingthe interference signal.

For another example and with reference to FIG. 7, for the case of anultrasound locating signal, we may define for each measuring cycle aperiod of a sensing window, only during which a pulse may be expected atthe receiver. This window, which represents the range of propagationtimes for all possible locations in the active area of the movabledevice, begins shortly after a timing pulse, e.g., the IR pulse, andends a certain time before the next timing pulse. The time periodsbetween successive sensing windows are guaranteed to be free of usefulsignal energy. All such empty periods are referred to herein asadaptation windows. Line 4 of FIG. 7 illustrates adaptation windows inthe case of an ultrasound receiver.

It is to be noted that, in certain systems, adaptation windows could bewidened, based on the continuity of position of the movable pointingdevice; since the reception times of successive pulses is thenpredictable within a range, e.g., a range proportional to the device'smaximum speed. The sensing window could then be narrowed accordingly andthe adaptation window could be widened commensurately.

In different embodiments, other characteristics of the interferingsignals may be assumed. These assumptions include:

(a) Interference signals mainly contain a single frequency orfrequencies within a very narrow band. Such a range is, at least afterappropriate filtering, close to the carrier frequency of the usefulsignal.

(b) the signal is essentially stationary, i.e. varies relatively slowly,if at all, in frequency or amplitude.

Owing to these characteristics it is particularly practical to generatean estimated interference signal, to serve for noise reduction in areceived signal during any given period, that is based on, or derivedfrom, the same signal some time earlier, i.e., a delayed version of thesignal. The estimated interference signal can thus also be regarded as aprediction (“a predicted estimate”) of the noise or interferencecomponent. Application of this principle in two specific embodimentswill now be described.

FIG. 5A depicts schematically a second embodiment of the invention,which is a first version based on the time shift principle discussedabove. It is similar to that of FIG. 4A in that it includes a summingdevice 33, whose one input is fed the normal received signal denotedy(k) and whose other input is fed the output of an adaptive filter 44,in this embodiment preferably including a FIR filter. Again, there is anadaptation loop from the output of the summing device 33, through anadaptive weight calculator 46, to the weights of the adaptive filter 44.However, unlike the embodiment of FIG. 4A, the input to the filter 44 istaken from the same received signal y(k) that is fed to the summingdevice 33. A switch control 54 continuously determines adaptationwindows, based on information received from the system, such asoccurrence of timing pulses and, optionally, of received locatingpulses. The switch control is coupled to the adaptive filter anddetermines when adaptation may and may not occur.

FIG. 5B shows one implementation of the embodiment of FIG. 5A. Theinterference signal is adaptively filtered by a FIR adaptive filter 44of the adaptive filter unit 52 by passing the input received signal y(k)through a delay module 58 that delays the input by an amount D timesample units. The switch controller 54 is shown in FIG. 5B ascontrolling a switch 56, which opens or closes the adaptation loopdepending on whether there is or is not adaptation. The adaptationmethod, realized in adaptive weight calculator 46, is preferably similarto that stated for the first embodiment, except that the term r(k)representing the separately received interference signal samples is hereequal to y(k−D), i.e. the received signal y(k) after a delay of D sampletime units. The adaptive FIR filter 44 is realized in one embodiment asshown in FIG. 4B.

Operation of this second embodiment of FIG. 5B is as follows: The switchcontrol 54 repeatedly determines adaptation windows, based oninformation received from the system, such as occurrence of timingpulses and, optionally, of received locating pulses. During eachadaptation window it causes the loop switch 56 (for the version of FIG.5B) to be closed, thus enabling the adaptation loop. During this period,the delayed version of the received signal passes through the filter 44and, as modified by the filter, is applied to the summing device 33,where it is subtracted from the currently received signal. Thedifference signal is applied to the adaptive weight calculator 46, whichmodifies the weights of the filter so as to reduce the difference signalat the next iteration, e.g., the next sample time. Since, during theadaptation window, the received signal presumably contains only theinterference component, the effect of the iterative adaptation will beto reduce the difference to zero, or nearly so; in other words, when thefilter becomes fully adapted, its output signal will nearly equal thenoise component in the currently received signal. At the end of theadaptation window the switch 56 opens and the weights of the FIR filterremain fixed at their last values. Received signal continues to be fedthrough both inputs to the summing device 33, where the two versionscontinue to be subtracted from each other. Now when a pulse of theuseful signal is received, it is immediately applied to the positiveinput of the summing device, while the negative input is fed anadaptively filtered earlier received signal, which does not yet containthe pulse. Because of the assumed stationarity of the interference, thefiltered signal continues to be a close approximation of the noisecomponent in the currently received signal and thus fully cancels it inthe summing device, resulting in the output (difference) signal toconsist almost entirely of the useful component alone.

It is noted that the value of the delay, D, and the timing and length ofthe adaptation period, as well as the factor μ in the adaptationformula, are parameters that may be chosen to optimize operation, basedon system characteristics and signal levels. It is also again noted thatthe blocks “delay” 58, “FIR filter” 44, “adaptive weight calculator” 46and “switch control” 54 in FIG. 5B collectively constitute, in effect,any one of the blocks generically named “Noise Estimator” 35 in FIG. 3.

Reference is now made to FIG. 6, which depicts schematically a thirdembodiment of the invention, which is a second embodiment based on thetime shift principle discussed above. It is particularly suitable forcases in which the interfering signal, after any input filtering, iscomprised essentially of a single or a small number of frequencies and,again, varies relatively slowly. It consists, in addition to the summingdevice 33, of an adaptive analyzer 62, a signal generator 64 and aswitch control 54. The case of the input, after appropriate filtering,including essentially one main frequency is described. The analyzer 62is fed the received signal and functions to continuously determine itsfrequency, phase, e.g., phase relative to a given time base, andamplitude, as well as their respective rates of change. It passes themeasurements on to the signal generator 64, which generates a sinusoidalsignal accordingly and feeds it to the summing device 33, where it issubtracted from the current received signal. In one embodiment, thecombination of the analyzer and signal generator is implemented asdigital phase locked loop circuit, well known in the art. The analyzeris active only during adaptation windows, as defined above. Toward theend of an adaptation window the analyzer calculates the three parametersfor a certain time ahead, akin to the delay of the second embodiment,based on the measured rates of change, and these parameters are used bythe signal generator during the ensuing sensing window. The switchcontrol module 54 controls this cyclical operation of the analyzer. Theeffect of operation of this embodiment is very similar to that of thesecond embodiment.

For the case that the interfering signal contains a plurality ofdistinct frequencies, the analyzer 62 may be modified to analyze each ofthe frequencies and to supply corresponding sets of parameters;similarly, the signal generator 64 may be modified to generate aplurality of sine waves accordingly.

In an alternate embodiment, to use an adaptive filter based on the LMSor other derivative method, a FIR filter whose weights are computedaccording to a least squares criterion is used. The weights arecalculated so as to minimize the least squared sum of a number M ofoutputs. Thus, denoting by e(k) is the vector of the M error signals,i.e.,e (k)=[e(k), e(k−1), . . . , e(k−M+1)]^(T),

where [ ]^(T) represents the matrix transpose, the criterion is todetermine the weight vector w for the FIR filter that minimizes the sumof the squares of e(k)'s, being Σ_(i=0 to M−1)[e²(k−i)]=e(k)·e(k).

Recall e(k)=y(k)−w·r(k), wherey (k)=[y(k), y(k−1), . . . , y(k−M+1)]^(T)r (k)=[r(k), r(k−1), . . . , r(k−M+1)]^(T).

For the moment, assume the weights do not change in time. This equationis representable as a set of M linear equations, which in turn isrepresentable as a linear matrix equation. Denoting by A(k) the M by Mmatrix of the equation for the M values k=0, 1, . . . , M−1, those inthe art will know that the least squares solution that minimizese(k)·e(k) is obtained by using the pseudoinverse of the matrix, i.e.,(A^(T)A)⁻¹A^(T).

In one embodiment, the weights are determined at a time when it is knownthat the reference signal r(k) input into the filter is known not tocontain components of the desired signal. This least squares FIR filtermethod is applicable to both the method that uses a separate referencesignal receiver, and for the method wherein the reference signal is aversion of the input signal when the desired signal is known not to bepresent.

For example, in one embodiment, when r(k) is a delayed version of y(k),andd(k)=w(125)y(k−125)+w(123)y(k−123)+w(121)y(k−121)+w(119)y(k−119),theny(k)−w(125)y(k−125)+w(123)y(k−123)+w(121)y(k−121)+w(119)y(k−119)=e(k),

then the equations to solve for w(125), w(123), w(121), and w(119) areA(k) w=y (k), where, for M values

${A(k)} = \begin{bmatrix}{y\left( {k - 125} \right)} & {y\left( {k - 123} \right)} & {y\left( {k - 121} \right)} & {y\left( {k - 119} \right)} \\{y\left( {k - 125 - 1} \right)} & {y\left( {k - 123 - 1} \right)} & {y\left( {k - 121 - 1} \right)} & {y\left( {k - 119 - 1} \right)} \\{y\left( {k - 125 - 2} \right)} & {y\left( {k - 123 - 2} \right)} & {y\left( {k - 121 - 2} \right)} & {y\left( {k - 119 - 2} \right)} \\\vdots & \vdots & \vdots & \vdots \\{y\left( {k - 125 - M + 1} \right)} & {y\left( {k - 123 - M + 1} \right)} & {y\left( {k - 121 - M + 1} \right)} & {y\left( {k - 119 - M + 1} \right)}\end{bmatrix}$ ${\underset{\_}{w} = \begin{bmatrix}{w(125)} \\{w(123)} \\{w(121)} \\{w(119)}\end{bmatrix}},{{{and}\mspace{14mu}{\underset{\_}{y}(k)}} = {\begin{bmatrix}{y(k)} \\{y\left( {k - 1} \right)} \\{Y\left( {k - 2} \right)} \\\vdots \\{y\left( {k - M + 1} \right)}\end{bmatrix}.}}$

Each element of (A^(T)A)⁻¹ and of A^(T) y(k) may then be computed assums. The linear equations may then be solved using many numericalmethods, e.g., Gaussian elimination.

In an improved least squares version, the weights of the filter aretaken as a moving average of the last few corresponding weights and thenewly calculated weight, such that the weights are updated from time totime.

For example, in one version,

${\begin{bmatrix}{w(125)} \\{w(123)} \\{w(121)} \\{w(119)}\end{bmatrix}({new})} = {{\begin{bmatrix}{w(125)} \\{w(123)} \\{w(121)} \\{w(119)}\end{bmatrix}({previous})} + {\begin{bmatrix}{w(125)} \\{w(123)} \\{w(121)} \\{w(119)}\end{bmatrix}{({calculated}).}}}$

The embodiments disclosed herein are in terms of digital techniques andare operative on digitized signals. As noted above, similar analogembodiments are generally possible but digital ones are preferable, asthey are more practicable. Specific realizations may be any ones knownin the art, including special-purpose digital circuits, assembled logicmodules and a set of instructions in a programmable digital processor orprocessing system, as well as any combination between them. Programmabledigital processors may include, in particular, so-called digital signalprocessors (DSP). In the case of a programmable processor it may beadvantageous to realize the modules of the present invention within thesame processor that serves for realizing the locator.

One embodiment of the invention performs the interference reductionusing the adaptive filers on only the ultrasonic received signals. Suchan embodiment is shown in simplified block diagram form in FIG. 5A. Thegeneral architecture used is that shown in FIG. 1C. The ultrasound andthe IR are all samples with an ADC at 680 KHz, i.e., with a period ofabout 1.47 μs. Because the IR pulses are received before the ultrasoundpulses, only one ADC is used to digitize both the IR and one of theultrasound channels, with a switch switching over to the ultrasoundchannel as soon as the ultrasound pulse has been sent. Referring to FIG.8A, the DSP memory has a set of IR received samples and two sets ofultrasound received samples, and processes these accordingly. In oneembodiment, adaptive interference reduction is carried out on only theultrasonic samples. Note that one alternate embodiment includesinterference reduction also for the IR channel, while yet anotheralternate embodiment includes interference reduction for the IR channel,but not for the ultrasound received signals.

In one embodiment, the samples from each of the ultrasound receivers aresubject to DC removal (clamping) by DC removal units 82 and 83 usingmethods known in the art. These DC-removed samples are then subject tointerference reduction using the structures 84 and 85, that may beimplemented as described above. After interference reduction, thesignals are each subject to a bandpass filter 86 and 87 for thefrequency range of interest. In one embodiment, the ultrasoundtransducers on the transmitting pointing device and the matchingreceives have a resonant frequency of 40 kHz, so that the bandpassfilters 82 and 83 are centered around this frequency. The pulse rate isabout 70 pulses per second. The signals after filtering are stored inmemory 88 for further processing, e.g., by position calculation method(shown as block 89) as described in principle above, and in oneembodiment, as in the Wood, et al. incorporated herein by reference U.S.Pat. No. 6,335,723 and in another embodiment, as in the Wood, et al.incorporated herein by reference U.S. Pat. No. 6,414,673.

FIG. 8B shows one embodiment of the adaptive interference reductionsunits 84 and 85, e.g., unit 84 for the first ultrasonic signal. Theadaptive filter includes a tapped delay line that provides a delay of121 units. The FIR filter whose coefficients are adaptively modified areof taps 121, 123, 125, and 127, thus are two samples apart. Theweighting and add accumulation calculations are adaptively adjusted byweight calculator 46. The switch control 54, together with switch 56,ensure that the adapting only occurs when we know there is no ultrasoundsignal, but only interference present.

As will be appreciated by those skilled in the art, the presentinvention may be embodied as a method, an apparatus such as a specialpurpose computing apparatus or a data processing system, or a carriermedium, e.g., a computer program product. The carrier medium carries oneor more computer readable code segments for controlling a processingsystem to implement a method. Accordingly, aspects of the presentinvention may take the form of a method, an entirely hardwareembodiment, an entirely software embodiment or an embodiment combiningsoftware and hardware aspects. Furthermore, the present invention maytake the form of carrier medium (e.g., a computer program product on acomputer-readable storage medium) carrying computer-readable programcode segments embodied in the medium. Any suitable computer readablemedium may be used including a magnetic storage device, such as adiskette or a hard disk, or an optical storage device such as a CD-ROM.

It will be understood that the steps of methods discussed are performedin one embodiment by an appropriate processor (or processors) of aprocessing system (e.g. computer) executing instructions (code segments)stored in storage. It will also be understood that the invention is notlimited to any particular implementation or programming technique andthat the invention may be implemented using any appropriate techniquesfor implementing the functionality described herein. The invention isnot limited to any particular programming language or operating system.

Reference throughout this specification to “an embodiment” means that aparticular feature, structure or characteristic described in connectionwith the embodiment is included in at least one embodiment of thepresent invention. Thus, appearances of the phrases “in an embodiment”in various places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures or characteristics may be combined in any suitable manner, aswould be apparent to one of ordinary skill in the art from thisdisclosure, in one or more embodiments.

Similarly, it should be appreciated that in the above description ofexemplary embodiments of the invention, various features of theinvention are sometimes grouped together in a single embodiment, figure,or description thereof for the purpose of streamlining the disclosureand aiding in the understanding of one or more of the various inventiveaspects. This method of disclosure, however, is not to be interpreted asreflecting an intention that the claimed invention requires morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the claimsfollowing the Detailed Description are hereby expressly incorporatedinto this Detailed Description, with each claim standing on its own as aseparate embodiment of this invention.

Thus, while there has been described what is believed to be thepreferred embodiments of the invention, those skilled in the art willrecognize that other and further modifications may be made theretowithout departing from the spirit of the invention, and it is intendedto claim all such changes and modifications as fall within the scope ofthe invention. For example, any formulas given above are merelyrepresentative of procedures that may be used. Functionality may beadded or deleted from the block diagrams and operations may beinterchanged among functional blocks. Steps may be added or deleted tomethods described within the scope of the present invention.

1. A method for determining the location of a pointing device in thevicinity of a set of receivers able to receive one or more locatingsignals transmitted through air near a surface, the method comprising:(a) transmitting one or more ultrasound pulses and one or moreelectromagnetic signal pulses to or from the pointing device when thepointing device is near the surface, and as a result: (a1) receiving atone or more ultrasound receivers a respective ultrasound signal as aresult of a transmitted ultrasound pulse propagating through air betweenthe respective ultrasound receiver and the pointing device, or (a2)receiving at an electromagnetic signal receiver a respectiveelectromagnetic signal as a result of a transmitted electromagneticpulse signal, or (a1a2) both receiving at one or more ultrasoundreceivers the respective ultrasound signal and receiving at theelectromagnetic signal receiver the electromagnetic signal, wherein atleast one of the received signals includes a respective interferingsignal; (b) for at least one of the received signals that may include arespective interfering signal, determining a respective estimatedinterference signal indicative of the respective interfering signalincluded in the at least one signal received in step (a); (c) for atleast one of the received signals that includes a respective interferingsignal, determining a signal indicative of the difference between therespective signal received in step (a) and the respective estimatedinterference signal determined in step (b); and (d) using the signaldetermined in step (c) to compute the location of the pointing device onor near the surface.
 2. A method as recited in claim 1, wherein step (b)includes for the at least one received signals that includes therespective interfering signal, receiving a respective referencecorrelated with the respective interfering signal, and filtering therespective reference signal using a respective filter to produce therespective estimated interference signal.
 3. A method as recited inclaim 2, wherein each respective filter is respective adaptive filter.4. A method as recited in claim 2, wherein each respective filter is arespective filter defined by a set of one or more filter parameters, theset selected from a plurality of sets according to a selectioncriterion.
 5. A method as recited in claim 1, wherein step (b) includesfor the at least one received signals that includes the respectiveinterfering signal, forming a respective reference signal from therespective signal received in step (a) at a time when the signalcorresponding to a respective transmitted pulse is not expected to beincluded in the respective received signal, and filtering the respectivereference signal.
 6. A method as recited in claim 5, wherein the formingof the respective reference signal includes delaying the respectivesignal received in step (a).
 7. A method as recited in claim 5, whereineach respective filter is a respective adaptive filter, and the adaptingby the respective adaptive filter uses the respective reference signalwhen such respective reference signal is known not to include the signalcorresponding to a respective transmitted pulse.
 8. A method as recitedin claim 5, wherein said determining of an estimated respectiveinterference signal includes filtering the respective received inputwith a respective adaptive filter that adapts a respective set of filterparameters, and wherein adapting of the respective filter parametersonly occurs when the signal corresponding to a respective transmittedpulse is not expected to be included in the respective received signal.9. A method as recited in claim 8, wherein step (b) occurs for theultrasound signals emitted by the pointing device, wherein theinterference signal includes an interfering signal in the frequencyrange of the ultrasound pulses, wherein the electromagnetic signalpulses are emitted as timing pulses for the ultrasound pulses, andwherein the timing of when to carry out adapting uses the timing pulses.10. A method as recited in claim 9, wherein said determining arespective estimated interference signal includes predicting therespective estimated interference signal from said signal received viathe air between the pointing device and the respective receiver duringsome time preceding a corresponding one of said given time periods. 11.A method as recited in claim 1, wherein said step (b) includes for theat least one of the received signals that includes the respectiveinterfering signal, determining the respective estimated interferencesignal from the respective received signal, and wherein said step (b)includes delaying the respective received signal to produce a respectivedelayed version of the respective received signal, and determining therespective estimated interference signal from the respective delayedversion.
 12. A method as recited in claim 11, wherein said step (b)further includes for the at least one of the received signals thatincludes the respective interfering signal, adaptively filtering saidrespective delayed version by a respective adaptive filter, eachrespective adaptive filter carrying out a respective adaptation process.13. A method as recited in claim 12, wherein said respective adaptationprocess is halted substantially prior to performing step (d) prior towhen it is expected that the respective received signal includes asignal corresponding to a respective transmitted pulse.
 14. A method asrecited in claim 12, wherein said respective adaptation process uses arespective error signal indicative of the difference between therespective estimated interference signal and the respective signalconcurrently received via the air.
 15. A method as recited in claim 12,wherein said respective adaptation process uses the LMS method or aderivative thereof for the respective adaptation process using arespective error signal indicative of the difference determined in step(c).
 16. A method as recited in claim 1 wherein at least one of thereceived signals includes the ultrasound pulses, wherein theinterference signal includes an interfering signal in the frequencyrange of the ultrasound pulses.
 17. A method as recited in claim 1,wherein step (b) occurs for the ultrasound signals emitted by thepointing device, wherein the electromagnetic signal pulses are emittedas timing pulses for the ultrasound pulses, and wherein the interferenceincludes interference at a frequency within the frequency range of theelectromagnetic radiation.
 18. A method as recited in claim 1, whereinthe ultrasound and electromagnetic signal pulses are transmitted fromrespective transmitters attached to the pointing device.
 19. A method asrecited in claim 1, wherein the receiving is by two or more ultrasoundreceivers or electromagnetic signal receivers or their combinationthereof in fixed relative positions to the surface, and wherein steps(b), (c), and (d) are performed with respect to at least two of saidrespective receivers.
 20. A method as recited in claim 1, wherein thesignal corresponding to a respective transmitted pulse is received by arespective sensor attached to the pointing device.
 21. A method asrecited in claim 20, wherein the transmitting is by one or moreultrasound and/or electromagnetic pulse transmitters in fixed positionsrelative to the planar surface, each transmitter sending respectivelocating signals, and wherein steps (b), (c), and (d) are performed withrespect to at least two of the received signals.
 22. An apparatus todetermine the location of a pointing device on or near a surface in thevicinity of a set of receivers able to receive one or more locatingsignals transmitted through air, the apparatus comprising: a pointingdevice; at least one ultrasound transmitter configured to transmitultrasound pulses through the air near the surface; an electromagneticsignal transmitter configured to transmit electromagnetic signal pulses;one or more ultrasound receivers to receive a respective ultrasoundsignal transmitted from the at least one ultrasound transmitter; and anelectromagnetic signal receiver to receive a respective electromagneticpulse signal transmitted from the pointing device, wherein therespective receivers may receive in addition respective interferingsignals, the apparatus further comprising: for at least some of thereceivers, a respective filter accepting a respective reference signallikely to be correlated with the respective interfering signal, theoutput of the respective filter generating a respective estimatedinterference signal indicative of the respective interfering signalincluded in the respective signal received at the respective receiver; arespective subtractor coupled to the output of the respective filter andto the output of each of the at least some receivers to determine arespective signal indicative of the respective difference between therespective received signal and the respective estimated interferencesignal; and a location determiner coupled to the output of the eachsubtractor and using the respective signal indicative of the respectivedifference to compute the location of the pointing device on a or nearthe surface.
 23. An apparatus as recited in claim 22, wherein therespective reference signal is determined from the respective signalreceived by the respective receiver at a time when the signalcorresponding to a respective transmitted pulse is not expected to beincluded in the respective received signal.
 24. An apparatus as recitedin claim 23, wherein the respective reference signal is formed by aprocess that includes delaying the respective signal received by therespective receiver.
 25. An apparatus as recited in claim 23, whereineach respective filter is a respective adaptive filter, and the adaptingby the respective adaptive filter uses the respective reference signalwhen such respective reference signal is known not to include the signalcorresponding to a respective transmitted pulse.
 26. An apparatus asrecited in claim 23, wherein said respective filter is a respectiveadaptive filter that adapts a set of respective filter parameters, theadapting of the respective filter parameters only occurring when thesignal corresponding to a respective transmitted Pulse is not expectedto be included in the respective received signal.
 27. An apparatus asrecited in claim 26, wherein the at least one ultrasound transmitter andthe electromagnetic signal transmitter are on the pointing device suchthat the ultrasound pulses are periodically emitted by the pointingdevice, wherein the one or more ultrasound receivers include at leasttwo ultrasound receivers, wherein the interference signal received byeach ultrasound receiver includes an respective interfering signal inthe frequency range of the ultrasound pulses, wherein theelectromagnetic signal pulses are emitted from the pointing device astiming pulses for the ultrasound pulses, and wherein the timing of whento carry out adapting of the respective filter parameters uses thetiming pulses.
 28. An apparatus as recited in claim 27, wherein saidrespective filter determines a respective estimated interference signalby a process that includes predicting the respective estimatedinterference signal from said respective signal received via the airbetween the pointing device and the respective receiver during some timepreceding a corresponding one of said given time periods.
 29. Anapparatus as recited in claim 26, wherein the at least one ultrasoundtransmitter are on the pointing device such that the ultrasound pulsesare periodically emitted by the pointing device, wherein the one or moreultrasound receivers include at least two ultrasound receivers, andwherein the interference signal received by each ultrasound receiverincludes an respective interfering signal in the frequency range of theultrasound pulses.
 30. An apparatus as recited in claim 22, wherein saidrespective filter is to determine the respective estimated interferencesignal from the respective received signal, and wherein said respectivefilter further includes a delay delaying the respective received signalto produce a respective delayed version of the respective receivedsignal, such that the respective filter determines the respectiveestimated interference signal from the delayed version of the respectivereceived signal.
 31. An apparatus as recited in claim 30, wherein saidrespective filter includes a respective adaptive filter to adaptivelyfilter said respective delayed version, each respective adaptive filtercarrying out a respective adaptation process.
 32. An apparatus asrecited in claim 31, wherein said respective adaptation process ishalted substantially prior to said location determiner computing thelocation of performing step using the respective signal indicative ofthe difference, such that the respective signal indicative of thedifference used in the computing is prior to when it is expected thatthe respective received signal includes a locating signal.
 33. Anapparatus as recited in claim 31, wherein said respective adaptationprocess uses a respective error signal indicative of the differencebetween the respective estimated interference signal and the respectivesignal concurrently received via the air.
 34. An apparatus as recited inclaim 31, wherein said respective adaptation process uses the LMS methodor a derivative thereof using a respective error signal indicative ofthe difference output by the respective subtractor.
 35. An apparatus asrecited in claim 22, wherein the at least one ultrasound transmitter andthe electromagnetic signal transmitter are on the pointing device suchthat the ultrasound pulses are periodically emitted by the pointingdevice, wherein the electromagnetic signal pulses are emitted from thepointing device as timing pulses for the ultrasound pulses, and whereinthe interference includes interference received by the electromagneticsignal receiver at a frequency within the frequency range of theelectromagnetic radiation.
 36. An apparatus as recited in claim 22,wherein the at least one ultrasound transmitter and the electromagneticsignal transmitter are attached to the pointing device.
 37. An apparatusas recited in claim 22, wherein the wherein the at least one ultrasoundtransmitter include at least two ultrasound transmitters at differentlocations on or close to the surface, wherein the electromagnetic signaltransmitter is on or close to the surface, and wherein the one or moreultrasound receivers and the electromagnetic signal receiver areattached to the pointing device such that the transmitted ultrasoundpulses and the electromagnetic signal pulses are received by respectivereceivers attached to the pointing device.
 38. An apparatus to determinethe location of a pointing device in the vicinity of a set of receiversable to receive one or more locating signals transmitted through airclose to a surface, the apparatus comprising: a pointing deviceincluding an ultrasound transmitter configured to transmit ultrasoundpulses and an electromagnetic signal transmitter configured to transmitelectromagnetic signal pulses; two or more ultrasound receivers atdifferent locations at or close to the surface configured to receive arespective ultrasound signal transmitted from the pointing device and arespective interfering signal; an electromagnetic receiver to receive arespective electromagnetic pulse signal transmitted from the pointingdevice and a respective interfering signal; and a processor programmedto carry out a method including: accepting and filtering a respectivereference signal likely to be correlated with the respective interferingsignal, the respective filtering generating a respective estimatedinterference signal indicative of the respective interfering signalincluded in the respective signal received at each receiver; acceptingthe respective signal received at each receiver and the output of therespective filtering operation, and subtracting the accepted signals todetermine a respective signal indicative of the difference between therespective received signal and the respective estimated interferencesignal; and using each respective signal indicative of the difference tocompute the location of the pointing device on a surface near the set ofreceivers.